Peavey KB-1 DC on Output

[Neutronarmy]

Hello,

I'm working on repairing a Peavey KB-1 that was experiencing a lack of output. Signal tracing initially found signal where I expected it to be leading up to the power amp and I had DC on the output so I expected the LM1875 to be at fault. I replaced the LM1875 and I still have ~18-20VDC on the output. I have +18V on pin 1 and pin 4. Pin 5 is getting ~38V. If I remove C25 from the board the DC disappears so there doesn't seem to be some short that is putting DC on the speaker from another source. I swapped C25 to make sure the DC blocking cap wasn't acting sketchy. Any tips or suggestions?

KB1[1].pdf

Thanks!
Chris

[g1]

Is that with the speaker disconnected? If so, put a 1K resistor or some similar value across the speaker connections and see if the cap discharges.

[Neutronarmy]

That is with the speaker disconnected. I'll give that a shot and report back.

[Neutronarmy]

I threw a 1K resistor across the speaker leads and it seemed to discharge the cap. However, when the speaker was connected again I've got 17VDC across the speaker. Hmmmm...

[Jazz P Bass]

Make double sure that the Headphone jack works properly.

[g1]

Is the speaker still measuring it's resistance?
Did it make a loud pop and move out or in very noticeably?
It will measure some DC for the brief second the cap charges. If the speaker has gone open, or there is a broken connection somewhere, then the DC will stay there.
Try a small value 1/2Watt resistor (around 10ohms) in the same way you tried with the speaker. If there is really the DC there that you measured when the speaker was connected, it will fry the resistor pretty quick.

[Enzo]

Did you find 17v ACROSS the speaker? or did you find 17v to ground from the speaker hot lead?

In other words could R40 be open?

[Neutronarmy]

I finally had the chance to get it on the bench again. I believe when I was measuring 17V across the speaker I was accidentally disconnecting one of the speaker leads; I could not replicate that issue. With any load attached - 10R resistor, 10K resistor, speaker - the 17VDC disappears. When it is unloaded DC is present both on the speaker leads and on the headphone jack (measured from tip to sleeve [ground]). When I have the speaker connected I get a very loud pop and a consistent hum. I attempted to run signal through it in said state and it made a temporary loud burst of tone and then quite making sound altogether - no pop, no hum, nothing. I'm going to swap back to the original LM1875 to see if I killed the replacement. This is a really strange repair and I feel like I'm overlooking something crazy obvious; the amplifier is only a few dozen components total.

[52 Bill]

When I have the speaker connected I get a very loud pop and a consistent hum. I attempted to run signal through it in said state and it made a temporary loud burst of tone and then quite making sound altogether - no pop, no hum, nothing. I'm going to swap back to the original LM1875 to see if I killed the replacement.

The output chip is being run in single supply mode, meaning that the chip has to be biased to 18 volts in order to fool it into thinking that there is bipolar supply. There should be 18 volts dc on the output of U3 (pin 4) and that is why C25 is there to block the dc voltage from getting to the speaker or headphones.

C25 should discharge the initial charge voltage through the speaker load resistance. If the speaker or wiring or the headphone jack, open the connection of the output to ground, then the 18 volts will remain on the negative end of C25 because there is no discharge path for it to bleed off through.

Clean the headphone jack to be certain that the switch contacts are clean and making good connections to the speaker wiring.

Read the DC voltage to ground at pin 4 of the output chip. There should be 18 volts (or so) there. If there is more, then the chip may be bad. Also read the DC voltage at the input pin 1. Again there should be 18 volts there as the bias voltage.

[Enzo]

With the speaker disconnected you get voltage, but with the speaker in place there is no offset voltage. So what is the problem? Does it work and sound OK?